Field of the Invention
The present invention relates to a nanofiber sheet and a method of producing the sheet.
Description of the Related Art
In recent years, a nanofiber sheet typified by a polymer nanofiber sheet, the sheet being formed by integrating a plurality of nanofibers each containing a polymer and three-dimensionally entangling the nanofibers with each other, has been attracting attention as a material having a large specific surface area.
The nanofibers of a related-art nanofiber sheet are three-dimensionally entangled with each other, but the entanglement is based on only physical entanglement. Accordingly, the related-art nanofiber sheet has a low mechanical strength and tends to be weak against a tensile force and friction, and hence involves problems in practical use. In view of the foregoing, various approaches have been developed for improving the mechanical strength of the nanofiber sheet. In Japanese Patent Application Laid-Open No. 2011-214170, there is a disclosure of an approach involving performing a partial bonding treatment in which a thread-like nanofiber structural body formed by twisting a plurality of nanofibers is heated to partially bond the nanofibers to each other. According to Japanese Patent Application Laid-Open No. 2011-214170, a nanofiber structural body improved in strength can be obtained by the partial bonding treatment. An approach involving bonding portions where nanofibers are in contact with each other through the application of energy has also been developed. In Japanese Patent Application Laid-Open No. 2012-134135, there is a disclosure of an approach involving thermally fusing extra-fine fibers obtained by dividing a division-type composite fiber formed of an ethylene-vinyl alcohol copolymer and polypropylene, and synthetic short fibers to mold a nonwoven fabric. Further, in International Publication No. WO2010/027063A, there is a disclosure of an approach involving forming extra-fine composite fibers from a composite resin molded product and thermally bonding the extra-fine composite fibers to form a fiber assembly.
However, in the approach of Japanese Patent Application Laid-Open No. 2011-214170, it is difficult to control, for example, a temperature suitable for the performance of the partial bonding treatment for the nanofibers, and hence the bonding of the nanofibers does not progress or the nanofibers largely melt depending on their surrounding environment. When the bonding of the nanofibers does not progress, a strength which the nanofiber structural body is required to have is not obtained. On the other hand, when the nanofibers largely melt, surrounding nanofibers completely integrate with each other, and hence the diameter of the resultant fiber itself becomes several micrometers or more to reduce the specific surface area of the structural body. In addition, in the approach of Japanese Patent Application Laid-Open No. 2012-134135, a temperature condition for the molding of the nonwoven fabric is narrow, specifically ranges from 140° C. to 175° C. Further, the short fibers each having a large diameter are used in the nonwoven fabric to be used, and hence many large pore diameters are present in the nonwoven fabric. Accordingly, the nanofiber structural body obtained by the approach of Japanese Patent Application Laid-Open No. 2012-134135 is not suitable for the collection of a minute substance and hence does not adapt to an application as a filter. Further, in the approach of International Publication No. WO2010/027063A, a melt electrospinning method is adopted, but the extra-fine composite fibers spun by the method each have a large diameter and hence the area of a bulk portion produced when thermal bonding of the extra-fine composite fibers is large. Accordingly, large pore diameters are found here and there in the resultant fiber assembly. Accordingly, it is difficult for the fiber assembly obtained by the approach of International Publication No. WO2010/027063A to prevent the leakage of an extremely fine substance, though the assembly has a high air permeability.